Slab Cistern

Also known as a variety of cache structures. These are massive very simple looking structures. Their method of construction and who build them are not known. They are most well known in the The Raikirian Desert and are often a symbol of the region. Sven Sontarin has shown they show in many more places and are a lot more widespread than originally thought.
The composite material appears to be something of a concrete with multiple layers around a nested stick structure of Tealtcra laced beams. This creates a chamber that can make scrying easier, as well as enhacing some clerics spells or altering wizarding spells in interesting ways. Not all locations have the same effects and it is unclear if these are unintentional side effects or were designed into the buildings for a reason. Almost universally people feel a sense of calm and peace when resting in or near one of these cisterns. While this doesn't generally register on a detect magic sense, as it appears to be sublte and naturalistic in effect.

Purpose / Function

While sizes vary from 30 feet across and 20 feet high to 200 feet across and 60 feet high they all feature a large pool of clean water for drinking. Usually the pool has multiple sections, though the smallest only have a single pool.
The larger of the structures also have extra rooms in the corners.
It is theorized that they are more than practical but also might have been religeous in nature.

Alterations

Many of these structures have been added onto with side buildings or buildings on top, with many differing styles and purposes. The largest alterations would include a keep and castle built onto and through one the larger slab cisterns. 
The most common alteration would be to add small buildings for houses and supplies.

Defenses

The buildings themselves have very thick walls and roofs made of a composite but simple-looking superstructure.